This invention relates generally to the field of medical devices, and more particularly to an implantable medical device for analyzing and classifying tachycardia episodes.
The heart is generally divided into four chambers, two atrial chambers and the two ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers of the heart go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells are contracting to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chambers to fill the ventricular chambers. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.
Control over the timing and order of the atrial and ventricular contractions during the cardiac cycle is critical for the heart to pump blood efficiently. Efficient pumping action of the heart requires precise coordination of the contraction of individual cardiac muscle cells. Contraction of each cell is triggered when an electrical excitatory impulse (an xe2x80x9caction potentialxe2x80x9d) sweeps over the heart. Proper coordination of the contractual activity of the individual cardiac muscle cells is achieved primarily by the conduction of the action potential from one cell to the next by gap junctions that connect all cells of the heart into a functional system. In addition, muscle cells in certain areas of the heart are specifically adapted to control the frequency of cardiac excitation, the pathway of conduction and the rate of impulse propagation through various regions of the heart. The major components of this specialized excitation and conduction system include the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and specialized cells called Purkinje fibers.
The SA node is located at the junction of the superior vena cava and the right atrium. Specialized atrium muscle cells of the SA node spontaneously generate action potentials which are then propagated through the rest of the heart to cause cardiac contraction. This SA node region normally acts as the intrinsic cardiac pacemaker. The action potential generated by the SA node spreads through the atrial wall, causing the atrial chambers to contract and the P-wave of an electrocardiogram signal.
The AV node consists of small, specialized cells located in the lower portion of the atrial chamber. The AV node acts like a bridge for the action potential to cross over into the ventricular chamber of the heart. Once the action potential has crossed over to the ventricular chambers, the bundle of His carries the action potential to specialized cardiac fibers called Purkinje fibers. The Purkinje fibers then distribute the action potential throughout the ventricular chamber of the heart. This results in rapid, very nearly simultaneous excitation of all ventricular muscle cells. The conduction of the action potential through the AV node and into the ventricular chambers creates the QRS-complex of an electrogram signal.
During the cardiac cycle, the action potential moves in an antegrade direction, first causing the atrial chambers to contract and then causing the ventricle chambers to contract. When this occurs the depolarization of the atria is xe2x80x9cassociatedxe2x80x9d with the depolarization of the ventricle. However, there are cardiac conditions in which the depolarizations (i.e., contractions) occurring in one chamber of the heart are not associated with subsequent contractions occurring in another chamber of the heart. In these situations, the contractions of these regions of the heart are xe2x80x9cdisassociated.xe2x80x9d
The ability to identify and classify the cardiac depolarizations occurring during a cardiac episode, such as a tachycardia episode, as either associated and disassociated is important for directing any additional analysis of the cardiac episode and for directing the appropriate therapy to treat the cardiac episode. One situation where classifying atrial and ventricular contractions of a tachycardia episode as being either associated or disassociated is in the discrimination, or classification, of ventricular tachycardia episodes from supraventricular tachycardia episodes. The ability to accurately classify a ventricular tachycardia episode from a supraventricular tachycardia episode allows the mechanism of the tachycardia episode to be identified which helps greatly in directing appropriate therapy. A need, however, still exists for a reliable way of classifying the cardiac depolarizations occurring during cardiac episodes as either associated or disassociated.
The present subject matter allows for cardiac depolarizations sensed during a cardiac episode in different cardiac regions to be classified as either being associated or disassociated. In one embodiment, the present subject matter relies upon isolating cardiac depolarizations sensed during a tachycardia episode in windows. The cardiac depolarizations within the windows are then counted, and based on the number and the location of the cardiac depolarizations within the windows the association or the disassociation of the cardiac depolarizations occurring during the cardiac episode can be determined. The present subject matter, thus, provides for accurate classification of cardiac episodes as either associated or disassociated which allows the mechanism of the tachycardia episode to be identified and which helps greatly in directing appropriate therapy to treat the cardiac episode.
In one embodiment, the present subject matter provides for a system and a method in which one or more cardiac signals are sensed and analyzed during a tachycardia episode to classify primary and auxiliary depolarizations occurring during the episode as either associated or disassociated. In one embodiment, the primary and auxiliary depolarizations are any combination of atrial depolarizations and/or ventricular depolarizations.
During the tachycardia episode, a first time interval is positioned to surround each of one or more primary depolarizations. In addition to surrounding the one or more primary depolarizations, the first time interval also surrounds auxiliary depolarizations that occur in the first time interval. The auxiliary depolarizations occurring in the first time interval are then counted. Based on the number and the location of the auxiliary depolarizations counted in the first time intervals, the one or more primary depolarizations and the auxiliary depolarizations of the tachycardia episode are classified as disassociated or associated.
In one embodiment, the first time interval is a calculated from an average value (XXavg) and a standard deviation value (XXsd) of auxiliary cycle lengths measured between pairs of consecutively sensed auxiliary depolarizations in a measurement window interval. The first time interval is calculated using the formula (XXavgxe2x88x92Y*XXsd) where Y is a predetermined constant. In one embodiment, the auxiliary cycle lengths that were sensed and analyzed during the tachycardia episode to determine the first time interval and the associated primary depolarizations sensed during the measurement window interval are analyzed to determine whether the tachycardia episode is associated or disassociated.
In one embodiment, classifying the tachycardia episode as associated or disassociated includes counting a first number (K) of auxiliary depolarizations during a first-half of the first time interval for each of the one or more primary depolarizations. In addition to counting the first number (K), a total number (N) of auxiliary depolarizations are counted during the first time interval for each of the one or more primary depolarizations. A K/N value is then calculated and the one or more primary depolarizations and the auxiliary depolarizations of the tachycardia episode are then classified as disassociated or associated based on K/N. Alternatively, K is compared to threshold values Klow and Khigh to classify the primary and auxiliary depolarizations, where the primary and auxiliary depolarizations are classified as associated when K is less than or equal to Klow or greater than or equal to Khigh, and the primary and auxiliary depolarizations are classified as disassociated when K is between Klow and Khigh.
In addition to using a first time interval in classifying the tachycardia episode, a second time interval can also be used to surround each of one or more auxiliary depolarizations of the auxiliary depolarizations, where the second time interval surrounds primary depolarizations that occur in the second time interval. A first number of primary depolarizations are then counted during a first-half of the second time interval for the one or more auxiliary depolarizations. A total number of primary depolarizations are also counted during the second time interval for each of the one or more auxiliary depolarizations. Based on the first number of auxiliary depolarizations, the total number of auxiliary depolarizations, the first number of primary depolarizations and the total number of primary depolarizations a classification of the tachycardia episode is made. In one embodiment, once the classification is made, additional analysis procedures can be better directed in analyzing the tachycardia episode. In turn, this could lead to more appropriate therapy being used to treat the tachycardia episode.
These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.